0
$\begingroup$

I need to modify a chuck's jaws so they can handle high temperatures, but will not scratch the workpiece. (Glass). The load will be very small.

I will probably use aluminium jaws and either coat them with, or attach to them, this material.

Example items being held:

  • Borosilicate glass tube, 30mm long, 6mm diameter, 5g weight
  • Borosilicate glass tube, 50mm long, 30mm diameter, 50g weight

The tube will be held horizontally by the chuck, and should be supported such that the other end will not droop by more than 2mm. That end is held in a propane bunsen flame, on the edge of the hottest part of the flame: a cone 15-20mm height, with a base ~12mm, 1200-1400°c.

I do not have figures on what the chuck jaws will have to withstand, but the air passing over the tip of the jaws probably wouldn't be more than about 250°c, and the temperature of the glass at that point will be about the same.

Of issue might be the heat radiated from the flame - so a heat reflecting material will help.

Also, since the rest of the chuck will be barely above room temperature, it may help to have thermally conductive material to dissipate heat from the tips of the jaws to the chuck.

Plastics seemail like an obvious choice for softness, but high temperature plastics seem hard to find. Silicone (260°c) and PTFE (204°c) are barely temperature resistant enough, but so far seem like the best choice, since higher temperature polymers like Vespel (300-400°c) are very hard to come by.

What alternatives are there? Metals? Composites?

The only other approach I might use is a heat shield directly in front of the chuck, but it would be awkward, as it would have to fit quite snugly around the workpiece, different shields would be needed for different size workpieces, and would be time consuming if batches of workpieces need to be processed. I can't imagine how this could be automated for large batches. Even then, scratching the workpiece could be a problem.

$\endgroup$
  • $\begingroup$ It would be helpful if you would include details such as the actual temperature you need to withstand, the weight of the item being held, its surface irregularity and how much motion/error you can tolerate (e.g., durometer). Be specific, with numbers. $\endgroup$ – Dave Tweed May 2 '16 at 11:33
  • 1
    $\begingroup$ Make sure to keep the jaws clean of debris, as glass fragments, sand, etc can also potentially scratch workpieces even with soft jaws. $\endgroup$ – wwarriner May 2 '16 at 17:20
  • 1
    $\begingroup$ I would line my jaws with Dupont Kapton film using a high temp silicone adhesive to bond the Kapton to the jaws. You can buy Kapton from McMaster Carr I believe. $\endgroup$ – William Hird May 2 '16 at 17:56
4
$\begingroup$

Carbon fibre or kevlar textile might work as a soft covering on an aluminium backing plate

In terms of metals high purity annealed copper might be soft enough on it's own. You can also get self adhesive copper foil tape.

It may also be worth considering using semi-consumable sleeves or collets. For example a hardwood dowel, bored to the appropriate diameter with a couple of slits along its length might do the trick. Even if it chars a bit at the end they might last long enough to be worthwhile.

Disposable sleeves also have the advantage that there is less risk of picking up swarf or grit that could scratch the glass.

Similarly it might be worth looking at O rings as a cheaper way to protect the rod than manufacturing jaws from stock, or maybe even use PTFE tape.

It might also be worth trying some dipping the glass in a natural or synthetic rubber to protect it.

In terms of heat shielding maybe look at small, disposable aluminium pie tins you could just punch a hole in the middle and suspend it by the glass rod itself. this might give just enough protection to make an otherwise marginal material viable. Similarly blowing air across the chuck with a portable fan or shop vac on blow mode might do just enough.

Another possible option is Tufnol, this is a phenolic-resin-based composite, laminated with glass, paper, or other textiles and has somewhat better heat resistance than a lot of other composites and is reasonably cheap and available as sheet stock.

| improve this answer | |
$\endgroup$
1
$\begingroup$

If you are unsure exactly what heat the material will be exposed to I would begin with Silicone rubber. It is relatively cheap and should withstand temperatures up to 260 degrees Celsius with few complaints. Plus you can usually pick exactly what stiffness you desire.

The selection on McMaster Carr is pretty good and for just a few bucks you will know whether or not you need to go searching for a more exotic material.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.